This invention relates to nematic and nematogenic compounds and to nematic mixtures of these compounds with each other or with other conventional nematic or nematogenic compounds or both.
For uses of nematic compounds and compositions see, e.g., App. Physics Letters 13, 46 (1968); Scientific American 222, 100 (1970) (dynamic scattering in electronic devices, construction and manufacture of such devices); furthermore: Zeitschrift fur Naturforschung 20a, 572 (1965); 23a, 152 (1968); Osterr. Chem.-Ztg. 68. 113 (1967) (use of nematic liquid crystals in nuclear resonance spectroscopy).
Nematic substances are compounds capable of forming an enantiotropically nematic phase, i.e., their conversion point from the anisotropic to the isotropic condition (transition point) is above their melting point. Substances which are monotropically nematic, i.e., wherein the conversion point from the anisotropic to the isotropic condition is below the melting point in the metastable range are called nematogenic. Furthermore, substances are nematogenic which form an enantiotropic nematic phase only in a mixture with other nematogenic or nematic compounds.
In the development of electronic components, particularly electronic indicating devices, characterized inter alia, by a flat structure and richness in image contrast, compared, for example, to conventional counter or cathode-ray tubes, liquid crystals with a nematic phase have served for several years as picture screen material. These compounds display, in their nematic range, i.e., the range between their melting point (m.p.) and their transition point (t.p.), a change in their light-scattering characteristic when can be controlled by electrical constant (d.c.) fields and alternating fields. A prerequisite of this property, called dynamic scattering effect, is that the dipole moment of the molecule forms an angle with the longitudinal axis of the molecule.
In order to exploit this dynamic scattering effect for the production of images, a thin layer of a thickness of a few microns of a suitable nematic compound is placed between two electrode plates, one or also both of which are transparent. If an electrical field is now applied, contrasts are produced by the change in light dispersion which can then be observed by direct frontal view or by rear viewing.
Nematic compounds, the dipole moment of which is disposed in the direction of the longitudinal axis of the molecule do not exhibit the dynamic scattering effect. When introducing such substances into an electric field, the molecules are aligned in parallel relative to the lines of flux. By dissolving foreign substances, e.g., dichroic or photochromic dyes, in such nematic liquids, the molecules of such dyes are aligned together along with the nematic carrier liquid by applying an electric field thereto. However, in the aligned condition, these dyes absorb less light than in the random condition. By the application of an electric field, a lightening of the color is effected between two transparent electrodes which, with a suitable choice of the dyes and their concentration, can go so far as to make the dielectric appear colorless. With nematic substances having a dipole moment in the direction of the longitudinal axis of the molecule in combination with appropriate dyes, e.g., methyl red or indophenol blue, it is thus possible to produce dielectrics which, upon being introduced between two conductive glasses, e.g., glass plates or panes, electrically controllable, wavelength-selective optical filters or shutters. Such devices are useful, for example, for cameras, laser outputs and automotive vehicle windows. A combination of three such devices which are transmissive, without an electric field, respectively for only one of the primary colors, viz., red, yellow or blue, and which can be electrically controlled individually, can be utilized for the reproduction of colored images or other information.
For the use of these effects in practice, nematic substances are required which are in the nematically liquid-crystalline condition at room temperature, since otherwise the indicating device must be thermostated, i.e., heated, which is expensive from a technical viewpoint. Nematic substances of this type are known, e.g., compounds of the asymmetrically p,p'-disubstituted benzylidene anilines and the asymmetrically p,p'-disubstituted azoxybenzenes classes.
A further requirement which must be met by practically usable nematic substances is a satisfactory chemical stability with respect to oxygen and moisture, particularly during the exposure to d.c. and alternating electrical fields. This condition is not met by the nematic compounds from the group of the benzylidene anilines. Such compounds are stable for only a few hours in a d.c. electrical field. In the form of Schiff bases, they are additionally split due to hydrolysis relatively readily by traces of moisture.
Nematic substances based on azoxybenzenes, although fulfilling the requirements of being chemically stable and exhibiting a low melting point, are limited in their practical usefulness due to their inherent yellow coloring resulting from their particular structure. Such substances are therefore unsuitable for the production of color effects. Also, the utilization of the dynamic scattering effect is not possible to an optimum extent with the azoxybenzene derivatives, because the thus-produced contrasts, due to the absorption of part of the impinging light by the colored nematic liquid, are, of course, weaker than those obtained when using a colorless nematic liquid.